In order to lengthen the life of devices in an organic EL display apparatus, structures in which organic EL devices are stacked have been proposed. Each of the organic EL devices generally has an organic layer including an emission layer and electrodes sandwiching the organic layer.
According to Japanese Patent Application Laid-Open No. 2005-174639, each pixel includes two sub-pixels. Each of the sub-pixels has a structure in which two organic EL layers having different luminescent colors are stacked to sandwich an electrode. The organic EL layer of an upper layer is provided with an emission layer which emits blue light, which is provided in every sub-pixel.
In generally, an organic EL device has a thickness equal to a wavelength of light and thus is greatly influenced by optical interference effects. When the emission layers of the respective organic EL layers emit lights, the lights are extracted to an outside after repetition of reflection, refraction, transmission, and absorption which are caused by differences between refractive indexes and absorption coefficients of the respective constituent layers. An amount of extracted lights is increased by constructive interference of lights passing through various paths.
The effect in view of interference effect of light is considered with regard to the configuration shown in FIG. 1.
FIG. 1 is a schematic view illustrating a case where a third organic EL layer 16 which emits blue light is stacked over a first organic EL layer 13 which emits green light, and a fourth organic EL layer 17 which emits blue light is stacked over a second organic EL layer 14 which emits red light. The third organic EL layer 16 and the fourth organic EL layer 17 are arranged side by side. Further, FIG. 1 shows a substrate 1, a first electrode (reflective electrode) 2a and 2b, a second electrode 5, and a third electrode 7. In view of an influence of interference, an interference effect between a direct light traveling from a light emitting position to a light extraction direction and light which is reflected on a reflective electrode and travels in the light extraction direction is maximum.
In this case, when the following relational expression (1) is satisfied, improvement of extraction efficiency due to interference is expected.2L/λ+δ/2π=m  (1)where L indicates an optical distance between an emission region of an emission layer and a reflection surface of the reflective electrode, λ indicates an emission spectrum peak wavelength of the emission layer, δ indicates a phase shift amount on the reflective electrode, and m indicates a natural number.
The expression (1) is derived from a constructive interference condition of an organic EL emission spectrum in a case of a resonance structure in D. G. Deppe, et al., Journal of Modern Optics, Volume 41, No. 2, p. 325-344 (1994).
The interference conditional expression (1) is used to obtain suitable optical path lengths in the light emission of the first organic EL layer 13 and the second organic EL layer 14 in a case of m=2. Results are illustrated in Table 1.
In Table 1, λ1 indicates an wavelength to be enhanced in the light emission of the first organic EL layer 13, and L1 indicates an optical distance between an emission region of the first organic EL layer 13 and the reflection surface of the reflective electrode 2a. 
In addition, λ2 indicates a wavelength to be enhanced in the light emission of the second organic EL layer 14, and L2 indicates an optical distance between an emission region of the second organic EL layer 14 and the reflection surface of the reflective electrode 2b. 
Further, λ3 indicates a wavelength to be enhanced in the light emission of the third organic EL layer 16. L3 indicates an optical distance between an emission region of the third organic EL layer 16 and the reflection surface of the reflective electrode 2a. L4 indicates an optical distance between an emission region of the fourth organic EL layer 17 and the reflection surface of the reflective electrode 2b. 
A phase shift on a reflection metal film is substantially π, and hence the calculation is performed on the assumption that δ is equal to π. It is suitable to make the wavelength to be enhanced equal to a photoluminescence (PL) peak wavelength because light emitting efficiency is improved. The PL is an emission spectrum generated in a case of photoexcitation.
The wavelength to be enhanced λ1 is set to 530 nm which is a PL peak wavelength of a green emission from the first organic EL layer 13. The wavelength to be enhanced λ2 is set to 630 nm which is a PL peak wavelength of a red emission from the second organic EL layer 14. The wavelength to be enhanced π3 is set to 450 nm which is a PL peak wavelength of a blue emission from the third organic EL layer 16 and the fourth organic EL layer 17.
As is apparent from Table 1, when the PL peak wavelength which is the emission wavelength to be enhanced changes, the suitable optical path length changes.
Further, Table 1 shows optimum interference distance at respective wavelengths derived from relational expression (1). In accordance with Table 1, setting optical distances of L1 to L4 in FIG. 1 are considered. L1 and L2 are 398 nm and 473 nm, respectively, since L1 and L2 are adjusted to the optimum interference distance corresponding to λ1 and λ2 shown in Table 1.
TABLE 1OptimumWavelengthInterference(nm)OrderDistance (nm)λ1530m2L398λ2630m2L473λ3450m3L563
Here, interference effect in the case where the third organic EL layer 16 and the fourth organic El layer 17 are formed in the same thickness is considered.
As is apparent from FIG. 1, the third organic EL layer 16 is stacked over the first organic EL layer 13 and the fourth organic EL layer 17 is stacked over the second organic EL layer 14. The third organic EL layer 16 and the fourth organic EL layer 17 are included in the two sub-pixels. Therefore, with respect to the optical distance between the emission region of the third organic EL layer 16 and the reflection surface of the reflective electrode, an optical distance L3 at a stacked position of the first organic EL layer 13 is different from an optical distance L4, which is between the emission region of the fourth organic EL layer 17 and the reflection surface of the reflective electrode, at a stacked position of the second organic EL layer 14.
For example, when L3 is set to 563 nm which is optimum interference distance, L4 is longer by 75 nm which is difference between L1 and L2.
As described above, the optical interference condition is changed by the difference between the optical distances. Thus, there arises a problem that the chromaticity of the light emitted from the third organic EL layer 16 is different from the chromaticity of the light emitted from the third organic EL layer 17.
On the other hand, in order that optical interference conditions are made equal and chromaticity is made equal in the third organic EL layer 16 and the fourth organic EL layer 17, it is necessary that thicknesses of the third organic EL layer 16 and the fourth organic EL layer 17 are changed and positions of emission region are made changed. In this case, the problem in that the film formation step comes to be complicated occurs.